The present invention relates to an ultrasonic rangefinder which is used to transmit and receive an ultrasonic wave signal for the detection of the distance from an object and, in particular, to improvements of an ultrasonic rangefinder for preventing a detour wave from reducing the accuracy of the detection.
There are already several ultrasonic rangefinders in existence, for example the one described in Utility Model Published Application No. Sho 57-68574. Such a device emits ultrasonic waves from an ultrasonic wave transmitter and receives pulses reflected from an object by an ultrasonic wave receiver. The distance is found from the phase difference between the emitted pulse and the received pulse.
As technically discussed in the art, received waves on a receiver include a detour wave which reduces the accuracy of detection. In a rangefinder, paticularly in a rangefinder which detects the vehicle height (the distance from the undersurface of the body of the vehicle to the ground), as disclosed in Japanese Patent Published Application No. Sho 57-182544, the reflected wave and the detour wave partially overlap, because the detected distance is short, (15 to 40 cm).
In a case of the type, the overlapping portions mutually interfere, causing the signal level to vary. In addition, when using the rangefinder on snow-covered roads and sandy soil, giving full consideration to the fact that the reflected wave signal is diminished because the ultrasonic wave signal is absorbed, there is a great necessity to set a low threshold level, and for this reason it is especially easy to produce the abovementioned error.
It has been proposed, as shown in Japanese Patent Published Application No. Sho 53-21953, to provide horns on a wave transmitter and a wave receiver for improving the directionality of the wave transmitter and receiver.
However, even with the improvement in directionality provided by the horns in such an ultrasonic rangefinder, when an ultrasonic transmitter and an ultrasonic receiver are installed in close proximity, a detour wave of an appreciable level which is directly incident from the wave transmitter and not reflected from the target object, is received in addition to a reflected wave.
This detour wave, especially when an object at a short distance is being detected (such as in the case of gauging the height of an automobile, as previously mentioned), partially overlaps the reflected wave and creates an obstruction, and is apt to cause a detection error by mixing with the reflected wave.
Furthermore, in a low temperature ambience the type of insulators normally used for damping the detour wave which transmits machanically from the transmitter to the receiver inside the rangefinder suffer a decrease in elasticity, and because their noise prevention effectiveness deteriortes, the level of the detour wave becomes higher than when at normal temperatures.